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You have to specify an onSubmit listener to return a false to stop the HTML posting to the server and then run the AJAX function for what ever process.

The form action should be a URL and not a call to a script.

Seems to me that you have hacked together a script with little understanding of the different technologies. If you look on this site, you will find lots of excamples of posting a forum by using AJAX. All it takes is a search here.